Certain agricultural machines, such as balers and forage harvesters, are required to pick up crop material that has been cut but still lies on the ground. The mechanism used to gather the crop material from the ground and feed it into the machine for processing is known as, and herein referred to as, a pickup. A pickup typically comprises a drum rotatably mounted on a frame and having radially projecting tines to drive the crop material upwards, past a guide (that is termed a windguard) and then into a feed channel through which the crop material is conveyed into the machine for processing. The construction of the pickup is not of importance to the present invention and has been described only by way of general background.
It is known for a pickup to be supported on two ground wheels and these may be adjustable in order to allow the ground clearance to be adjusted. As the ground wheels are provided only for support, and are not used for steering, they are commonly designed as caster wheels so that they align themselves automatically with the direction of travel of the machine. As is known, a caster wheel is pivotable about a generally vertical axis, generally known as and herein referred to as a caster axis, but the point of contact of the wheel with the ground is offset from the caster axis. As a result, drag rotates the wheel such that its point of contact with the ground always lines up behind the caster axis in the direction of travel thereby causing the wheel to point automatically in the correct direction.
A problem arises with this wheel design when the pickup is transported on public roads with the wheels raised off the ground. When they are not in contact with the ground, the caster wheels can rotate freely and can move to a position where they project laterally beyond the sides of the pickup. In such a position, they may increase the width of the pickup beyond legally permissible limits for transportation on a public highway.